Why a non-final “local” variable cannot be used inside an inner class, and instead a non-final field of the enclosing class can?

Question

There are some topics on Stack Overflow on the compiler error Cannot refer to a non-final variable message inside an inner class defined in a different method a

Answer 1

The value you use must be final, but the non-final fields of a final reference can be changed. Note: this is implicitly a final reference. You cannot change it.

private String enclosingClassField;
private void updateStatus() {
    final MutableClass ms = new MutableClass(1, 2);
    Runnable doUpdateStatus = new Runnable() {
         public void run() {
             // you can use `EnclosingClass.this` because its is always final
             EnclosingClass.this.enclosingClassField = "";
             // shorthand for the previous line.
             enclosingClassField = "";
             // you cannot change `ms`, but you can change its mutable fields.
             ms.x = 3;
         }
    }
    /* do something with doUpdateStatus, like SwingUtilities.invokeLater() */
}